1. Field of Invention
The present invention relates to a method for spinning nanofibers that combines aspects of electrospinning and melt-blowing, its application to spinning of hyaluronan and the nanofibrous materials made thereby.
2. Discussion of the Background
One technique conventionally used to prepare fine polymer fibers is the method of electrospinning. When an external electrostatic field is applied to a conducting fluid (e.g., a charged semi-dilute polymer solution or a charged polymer melt), a suspended conical droplet is formed, whereby the surface tension of the droplet is in equilibrium with the electric field. Electrospinning occurs when the electrostatic field is strong enough to overcome the surface tension of the liquid. The liquid droplet then becomes unstable and a tiny jet is ejected from the surface of the droplet. As it reaches a grounded target, the jet stream can be collected as an interconnected web of fine sub-micron size fibers. The resulting films from these non-woven nanoscale fibers (nanofibers) have very large surface area to volume ratios.
The electrospinning technique was first developed by Zeleny[1] and patented by Formhals[2], among others. Much research has been done on how the jet is formed as a function of electrostatic field strength, fluid viscosity, and molecular weight of polymers in solution. In particular, the work of Taylor and others on electrically driven jets has laid the groundwork for electrospinning[3]. Although potential applications of this technology have been widely mentioned, which include biological membranes (substrates for immobilized enzymes and catalyst systems), wound dressing materials, artificial blood vessels, aerosol filters, and clothing membranes for protection against environmental elements and battlefield threats[4-26].
The major technical barrier for manufacturing electrospun fabrics is the speed of fabrication. In other words, as the fiber size becomes very small, the yield of the electrospinning process becomes very low. For example, if one considers a polymer melt being spun from the spinneret with a diameter of 700 μm, and the final filament is formed with a diameter of 250 nm, the draw ratio will then be about 3×106. As the typical throughput of the extrudate from a single spinneret is about 16 mg/min (or 1 g/hr), the final filament speed will be about 136 m/s, as compared to the highest speed (10,000 m/min or 167 m/s) attainable by the high-speed melt-spinning process. Thus, the throughput of the spinneret in conventional electrospinning is about 1000 times lower than that in the commercial high-speed melt-spinning process.
Another major technical problem for mass production of electrospun fabrics is the assembly of spinnerets during electrospinning. A straightforward multi-jet arrangement as in high-speed melt spinning cannot be used because adjacent electrical fields often interfere with one another, making the mass production scheme by this approach impractical.
A unique esJets™ technology for multiple-jet electrospinning process has recently been developed for manufacturing of non-woven membranes having fibers with diameters in the tens of nanometers size range. Three patent applications based upon this technology have been filed[27-29] and several papers have also been published[30-34].
Hyaluronan (HA) is an associated polymer, having the following structure:

HA has an acidic group as well as a glucosamine segment. The presence of this weak acid makes the polymer a polyelectrolyte, i.e., its charge density depends on the degree of dissociation, that can be influenced by factors including, but not limited to:                pH        ionic strength        nature of co-ions and counter ions        solvent quality that shall also affect the above 3 conditions.        
The degree of association can be disturbed by physical and/or chemical means. For example:                By physical means, e.g., ultra-sonics, shear, microwave, etc.        By chemical means, such as complex formation with a liquid, e.g., polyethylene oxide is soluble in water because of its hydrogen bonding with water.        